The obtaining of undisturbed core samples has become increasingly wide spread for purposes such as the monitoring of well sites and for plotting the boundaries of contaminated ground in preparation for the clean up of environmental spills or in drilling for oil, natural gas or various minerals when samples of the material of a formation which is being drilled are needed.
The accepted means of acquiring a soil sample has been to slip a core barrel into the hollow part of a hollow-stem auger drill after the auger drill has drilled into the sampling area. In capturing a sample with this method, the virgin soil is greatly disturbed before the core barrel can be introduced into the sampling area.
The invention involves use of so called downhole percussion drilling techniques and apparatus used extensively for hard rock drilling and where relatively straight holes are required. As described in U.S. Pat. Nos. 3,517,754 and 4,084,646, in downhole drilling, the drill constitutes a compressed air actuated piston which comprises a percussion device which follows the drill bit down the hole being drilled. Air under pressure delivered from a compressor on the drilling rig is directed downwardly through the drill steel and channelled so that it initially and rapidly moves the piston upwardly away from the drill bit. When the piston is at the top of its stroke, air delivery passages are opened admitting the compressed air above the piston and uncovering an exhaust passage which allows a path for air to be exhausted out of an opening in the face of the drill bit. The power of the expanded compressed air is released upon opening of the exhaust passage forcing the piston rapidly downwardly onto a striking face of the drill bit. The impact of the piston against the drill bit drives the bit into the rock surface underlying the bit. The exhaust air then travels upwardly carrying broken rock out of the hole being drilled. Using equipment of the type described, holes can be drilled having a diameter of up to about 30". Smaller sized drill bits are utilized for drilling of wells, mining and the like. The larger sized holes offer a cost efficient mechanism for drilling of holes for footings, elevator shafts and conduits.